Hard materials obtained by coating TiC, TiCN, TiN, or Al2O3, on a surface of a substrate made of, for example, cemented carbide or cermet using a chemical vapor deposition (CVD) or a physical vapor deposition (PVD) are conventionally used in a cutting tool. Such a cutting tool is widely used in a variety of fields such as metal cutting work due to wear resistance of its coating layer(s) and toughness of its substrate.
However, there has been a problem that if such a cutting tool has weak adhesion between the substrate and the coating layer(s) deposited thereon, the coating layer(s) may be peeled from the substrate during the cutting process, and the cutting tool may rapidly wear down, reducing its lifetime. There has been another problem that if hard impact is applied to the cutting edge during the cutting process, the coating layer(s) may be peeled due to the weak adhesion between the substrate and the coating layer(s) deposited thereon, and the cutting tool may be fractured.
In order to solve the aforementioned problems, a method of improving adhesion between the substrate and the coating layer(s), in which the surface of the substrate is roughened to increase its surface area so as to obtain an anchor effect (refer to Patent Document 1), have been conventionally proposed. Also, there has been proposed a method of improving adhesion between the substrate and the coating layer(s) by diffusing constitutive elements of the substrate to the coating layer(s)(refer to Patent Documents 2 or 3).
Specifically, a first layer of TiC or TiN, a second layer of TiCN including columnar crystal, a third layer of TiC, TiCO or the like, and a fourth layer of Al2O3 are sequentially coated on the surface made of WC-based cemented carbide containing Co using the CVD method, in which components W and Co contained in the substrate are diffused to the first and second layers or to the first to third layers.    Patent Document 1: Japanese Patent Application Laid-Open No. 5-44012    Patent Document 2: Japanese Patent Application Laid-Open No. 7-243023    Patent Document 3: Japanese Patent Application Laid-Open No. 8-118105